Signatures of Incipient Jamming in Collisional Hopper Flows

TL;DR

This paper investigates how velocity fluctuations and their spatial heterogeneity signal the approach of jamming in granular hopper flows, revealing complex, rate-dependent fluctuation patterns and non-Gaussian velocity distributions near jamming.

Contribution

It introduces detailed analysis of velocity fluctuation signatures and heterogeneity in inhomogeneous hopper flows approaching jamming, using event-driven simulations.

Findings

Velocity autocorrelation times vary spatially and with flow rate.

Fluctuation relaxation times are longer at the center at high flow rates.

Non-Gaussian velocity distributions increase as flow slows.

Abstract

Many disordered systems experience a transition from a fluid-like state to a solid-like state following a sudden arrest in dynamics called jamming. In contrast to jamming in spatially homogeneous systems, jamming in hoppers occurs under extremely inhomogeneous conditions as the gravity-driven flow of grains enclosed by rigid walls converges towards a small opening. In this work, we study velocity fluctuations in a collisional flow near jamming using event-driven simulations. The average flow in a hopper geometry is known to have strong gradients, especially near the walls and the orifice. We find, in addition, a spatially heterogeneous distribution of fluctuations, most striking in the velocity autocorrelation relaxation times. At high flow rates, the flow at the center has lower kinetic temperatures and longer autocorrelation times than at the boundary. Remarkably, however, this trend reverses itself as the flow rate slows, with fluctuations relaxing more slowly at the boundaries though the kinetic temperatures remain high in that region. The slowing down of the dynamics is accompanied by increasing non-Gaussianity in the velocity distributions, which also have large spatial variations.